The present invention relates to a flow control valve for use in an antilock brake control device for a motor vehicle.
With the spread of antilock brake control devices for motor vehicles, it is becoming an urgent requirement to develop an antilock control device applicable to compact economy cars. To meet this requirement, it was proposed in GB 8512610 to use a single solenoid valve for each vehicle wheel and effect control with in two control modes, i.e. pressure reduction and slow pressure increase, instead of using two solenoid valves for each vehicle wheel as disclosed in Japanese Examined Patent Publication 49-28307.
The device disclosed in the former Publication is shown in FIGS. 4A, 4B and 4C in which a flow control valve 3 is employed to increase the braking pressure in a controlled manner in place of a solenoid valve as used in the latter Publication. This flow control valve 3 comprises a housing 31 formed with an inlet port 31a communicating with a master cylinder 2, an outlet port 31b communicating with a wheel brake 4 and a discharge port 31c communicating with a solenoid valve 5 serving as a discharge valve, and a spool 32 slidably mounted in the housing 31 and biased by a spring 34 to open and close the fluid communication among these ports.
When the valve 3 is in its original position shown in FIG. 4A, where the antilock control is not in action, a large-flow channel is formed extending from the inlet port 31a to the outlet port 31b through a peripheral groove 32a formed in the outer periphery of the spool 32. When the solenoid valve 5 is energized and opened to reduce pressure for effecting antilock control, hydraulic oil will be discharged through the discharge port 31c into a reservoir 63. This will move the spool 32 to the position shown in FIG. 4B owing to a difference in pressure at both ends thereof. In this state, the abovementioned large-flow channel is closed by an edge 32b on the spool 32.
The spool 32 will further move to the position shown in FIG. 4C where part of the peripheral groove 32a at the side of an edge 32c opens to a passageway 31e. Thus a discharge channel is formed from the outlet port 31b to the discharge port 31c through the groove 32a and the passageway 31e, allowing hydraulic oil in the wheel brake 4 to be discharged into the reservoir 63 through the solenoid valve 5 to reduce the braking pressure. The hydraulic oil discharged is suctioned and pressurized by a pump 61 driven by a motor 62 so as to be returned to the line between the master cylinder 2 and the inlet port 31a.
When the solenoid valve 5 is deactivated in the state shown in FIG. 4C to increase the braking pressure again, the spool 32 will perform a metering of the oil at its edge 32d, forming a restricted-flow channel connecting the inlet port 31a with the outlet port 31b through a passage 31d, an orifice 33, a pressure reducing chamber 36, a passage 31f, the passage 31e and the peripheral groove 32a. The wheel braking pressure will rise slowly. When the pressure difference between the inlet port 31a and the outlet port 31b reduces to a certain level, the spool 32 will return to its original position shown in FIG. 4A.
This arrangement is economical because each wheel is controlled with a single solenoid valve. The opening of a passage between the metering edge 32d and the passage 31f (hereinafter referred to as variable-size orifice) changes so that the flow rate during the reapplication of pressure during the antilock control will be constant, determined by the pressure difference at both ends of the orifice 33, which is determined by the effective sectional area of the spool 32 and the biasing force of the spring 34. This will not only serve to keep constant the flow rate through the orifice irrespective of the pressure difference between the inlet port 31a and the outlet port 31b, but will also make it possible to reduce the flow rate through the orifice even if it has a rather large diameter because the pressure difference at both ends of the orifice 33 can be limited to a minimum. Thus this system can be advantageously applied to a compact car having a small-sized brake which requires a small amount of hydraulic oil.
With a flow control valve of the type in which the communications among a plurality of ports are changed over by moving a spool, it will become impossible to change over the communication if the spool should get stuck owing to rusting.
Though the spool can get stuck at any point within the range of its stroke, let us assume now that the spool 32 has become stuck in the position shown in FIG. 4A. In this state the pressure on the wheel brake can be controlled because the large-flow channel extending through the peripheral groove 32a remains open though the antilock control is deactivated.
But if the spool 32 should become get stuck in the position shown in FIG. 4B, where the communication between the inlet 31a and the outlet 31b as well as the communication between the outlet 31b and the discharge port 31c are shut off, the brake fluid from the pressure source can be sent to the outlet 31b only through clearance formed between the spool and the housing. This will extremely worsen the controllability of pressure on the wheel brake.
If the spool 32 should become stuck in the position shown in FIG. 4C, where only a restricted-flow channel remains open, not only will the variable-size orifice become unadjustable, but also the brake pressure has to be applied to the wheel brake through the restricted-flow channel even during the normal braking mode where the antilock control is out of action. This may retard the rising of braking pressure because the flow rate is restricted excessively. Although the spool will rarely get stuck in such a position in practice, it is necessary to take some measures against this problem in view of the fact that safety is the most important factor with wheel brakes.